Method of securing adhesion of rubber to metal



ER To oct- 5 I Patented 715,- 1935 METHOD or saounme mansion or RUBBER 'ro METAL Henry R. Minor, Oak Park, 11]., assignor, by

mcsne assignments, to Industrial Process Corporatlon, of New Yo gratoga Springs, N. Y., a corporation Application March 10, 1934, Serial No. 714,920

23 Claims.

This invention relates to a method of securing adhesion between rubber and metal and more particularly between a relatively soft grade of rubber and ferrous metal.

5 Numerous methods have heretoforebeen de- 'vised for securing adhesion between rubber and metal. It is old, for instance, to vulcanize hard rades of rubber directly to both ferrous and non-ferrous metal. In the case of soft grades of rubber, however, direct vulcanization has proved unsatisfactory. Although much better results:

have been obtained, when using soft rubber, by the employment of cement prepared from rubber isomers, even this method has its drawbacks. Other expedlents have included the use of brass plating over ferrous metals to aid in the adhesion between the rubber and metal surfaces.

I have now discovered that relatively soft grades of rubber may be bonded directly to ferrous metals and their alloys in a simple and economical manner and adhesions secured that are as good and better than those obtained by the use of cement. The simplicity and adaptability of my method for numerous uses makes it possible to manufacture many articles combining rubber and metal which have not heretofore been ca.- pable of successful accomplishment. In particular, my method has considerable value in corn nection with the cushioning of members subjected to shock and capable of reIativeimovement, as in engines andother parts in automotive vehicles.

As contrasted with the adhesion secured by other methods, the adhesion between rubber and metal obtained according to my method is capable of withstanding severer flexing stresses at higher temperatures. Furthermore, the rubber, after union with the metal, exhibits a set that is well within the usual tolerances. By this is 40 meant that the rubber does not assume an excessive permanent set when'subjected to compression but, after the compressing force is released, returns to substantially its original thickness.

It is therefore an object of this invention to provide a method of securing good adhesion between rubber and metal and particularly between the softer grades of rubber and ferrous metals and their alloys.

It is a further important object of this invention to provide a relatively simple and economical method of making composite articles and-,the like by the adhesion of rubber to metal without the necessity of employing a contact layer of hard rubber or special rubber cement. 5

It, is a further important object of this invention to provide a method of securing a high degree of adhesion between rubber and metal that will withstand severe flexing stresses and that will have excellent tensile strength, good 10 resistance to high temperatures and a high degree of permanency. I

It is a further important object of this invention to provide a method of bonding relatively soft rubber to ferrous metal andferrous 15 alloys that includes a simultaneous vulcanizing of the rubber compound to produce a soft vulcaniz'ed rubber of satisfactory physical qualities, including that of possessing a relatively low vminimum set.

Other and'further important objects of this invention will become apparent from the disclosures-in the specification and the accompanying 1 drawing.

This invention (in a preferred form) is illusg trated in the drawing and hereinafter more fully described. t On the drawing:

Figure 1 is a fragmentary sectional view of a composite sheet or plate, illustrating an embodiso ment of this invention in the bonding of sheet rubber to a metal plate. 1

Figure 2 is a fragmentary sectional view of the composite sheet or plate of Fig. l, with an additional layer of rubber vulcanized thereto.

Figure 3 is a top plan view of a composite disk of metal and rubber, and

Figure 4 is a sectional view taken substantially on the line IV-IVof Fig. 3. t

As shown on the drawing:

The reference numeral it] indicates a sheet or plate of a ferrous metal or ferrous alloy' having applied thereto a sheet ii of a relatively soft grade of rubber. Although myinvention is applicable to both ferrous and non-ferrous metals, as the most important advantages asnow contemplated, result from the bonding of rubber directly to ferrous metals and their alloys without the use of special expedients, such asscoring or grooving the surface of themetal, or employing spe- 50 T v om n.

temperatures, which are notably higher than the V cial rubber cement. However, results may be ob tained with my process on brass plated metal that are superior to the results heretofore realized by the usual rubber adhesion processes.

Similarly, although my invention will be specifically described with respect to the bonding of relatively soft grades of rubber to metal, it 1 will be understood that this is because the advantages of my invention are most completely realized in the use of the softer grades of rubber rather than the harder grades. Ordinary methods of vulcanizing hard rubber to metal are reasonably satisfactory but are not at all satisfactory for securing soft rubber to metal.

In accordance with my invention, the layer H of vulcanizable soft rubber compound may either be applied to the metal plate l0 without being pressed thereagainst with any considerable force} dium may be applied directly to the surface l3 T or indirectly thereto through contact with a heat ed metal wall. Where steamis used, the temperatures produced by steam pressures of pounds per square inch and over, and'preferably about pounds per square inch, have proved very satisfactory for obtaining goodadhesion between the rubber layer II and the metal plate l0. The'heating time to be employed wilLof course, depend in part upon the composition of the-rubber layer II and also upon its thickness, longer time periods being used with greater thicknesses of the rubber layer ll. Steam pressures over lbs. per sq. in. have been satisfactorily used, as have also heating times of less than one hour. 1

As a result of the use of these relatively high usual vulcanizing temperatures, the rubber locally of the interface I! is depolymerized to a consid erable extent and its viscosity correspondingly lowered. By the term "depolymerization is meant the phenomenon that occurs in the structure of the rubber molecule and believed to result in the shortening of the molecular chain.

At the same time, the heating of the metal plate l0 causes it to expand and enlarges the pores of the surface of the metal in contact with the rubber layer ll. As a result, it is believed that the depolynierized and therefore less viscous rubber physically penetrates into the surface of the metal, even if only for microscopical distances, to secure true adhesion to the metal.

The time during which heat is applied to the metal plate II in order to secure good adhesion between the rubber layer II and the metal plate III .will vary in accordance with the temperatm'o used and also with the thickness of the rubber sheath I I. with a steam pressure of about 135 lbs. per sq. in. and a rubber sheath or layer of from 1 to fiths of an inch in thickness, the time may be as short as one hour or even somewhat shorter. With lower steam pressuresand thicker rubber layers, the time may have to be increased to a matter of several hours.

The conditions to be employed for securing good adhesion between the rubber and metal, and the results obtained, will obviously depend consider ably upon the composition of the rubber compound used in preparing the layer II. The following will serve to illustrate various typical formulae for the rubber compound:

Formula No. 1

, 4 Partsby wt. 5 Smoked sheets 100 Methylene p-toluidine 2.6 3 Sulfur 6.5 Zinc oxide 5 Aldol-alpha naphthylamin 1.75 Soft car 80 Lime 4 7 Formula No. 2 Smoked she 5o 15 ,Rolled brown 50 Methylene p-toluidine 2.6 Sulfur 6.5 Zinc oxide- 35 '20 China clay. 28.5 Aldol-alpha naphthylamine 1.75 Lime 1.:

Formula No. 3 Smoked sheet 100 25 Methylene p-toluidine 2.6 Sulfur 6.5 zinc mrirlp 5' Aldol-alpha naphthylamine 1.75 30 Carbon black 45 Lime 4 Formula No. 4

Smoked sh Methylene p-toluldine 2.6 Sulfur 6.5 Zinc oxide 5 Aldol-alpha naphthylamine 1.75 Soft carbon Y 45 .Lime 4 The following table will indicate the results obtained by the use of the foregoing formulas, showing the hardness of the rubber compound as obtained on a Stlckney durometer, the step expressed in percentages and the quality of the As will be observed, a relatively low minimum set and excellent adhesion may be secured by my method with rubberjcompositions varying in hardness over a considerable range. In fact, there is no difliculty in securing excellent adhesion between the rubber and metal whether the rubber stock be soft, such as indicated by a hardness of 30 to 40 units, or whether it has a hardness of between 50 and 60 units. In the past, it has been practically impossible by the use of other meth-'- ods to secure satisfactory adhesion with a hsrd'-' ness'of60. v

The method by which the percentage of set is obtained is as follows. The sample of rubber to be tested should be a disc 1%" in diameter and b inch-thick. The tester used has a pressure foot 7;" in diameter, by means of which a spring load of 400 lbs. is applied to the surface of the rubber 15 not more than 10% set on a 40 to 60 hardness of rubber. As indicated in the above table, the set of rubber bonded to metal in accordance with my invention is in general within these permissive ranges. My invention is, of course, not limited to the 'use of rubber stock having the specific formula: given above, since, in general, any vulcanlzable rubber stock capable of producing a soft rubber .upon vulcanization can be employed. By

the term "soft rubber is meanta rubber having a durometer test between about 20 and 80 units on 'a Stickney .durometer. There are, however, certain points to be observed in compounding the rubber stock for use in carrying out my invention if satisfactory results are to be obtained.

In the first place, it is preferable to use an amount of sulfur somewhat in excess of that ordinarily used for vulcanizing purposes if excellent adhesion is to be obtained. This excess of sulfur, however, does not show up in the vulcanized rubber as free sulfur but appears to be more efliciently combined with the rubber than would be the case if the rubber stock were subjected merely to the ordinary vulcanizing temperatures and pressures. This phenomenon can possibly be explained by reference to the higher temperatures that are employed in my-process. It is my theory that the high temperature to which the metal plate It is heated causes the sulfur to be driven away from the interface 52 because of the fact that the sulfur in the rubber locally of the interface i2 will-bec0me heated to substantially the temperature of the metal plate it and a relatively high sulfur vapor pressure-thereby built up, with the result that there will be a flow of sulfur or sulfur vapor toward the inner parts of the rubber layer ii.

- As a consequence of this temporary driving away of the sulfur from the interface ii, the rubber adjacent the interface i2 is quickly depolymerized upon the application of heat through the metal plate it and depolymerization proceeds to thenecessary extent to provide good adhesion between the. rubber and metal before vulcanization of the rubber to any substantial extent can occur. Later on, as the heating is continued, the vulcanization of the body of the rubber in the layer i l away from the interface is completed, the

entire mass of the rubber in the layer I i is brought to substantially the same temperature throughout,

- and the vapor pressure of the sulfur becomes sub 75 9 ber'layer.

stantially equalized at all points in the layer ii.

Consequently, even the rubber adiacent the interface 82 is ultimately vulcanized to substantially the same degree of vulcanitation as the rubber in other parts of the layer' 8 i. It is probable that owing to the higher temperatures used in my process than those customary in vulcanizing processes,

' the sulfur is taken up more completelyby the rubber and more sulfur is consumed than in the or-' dinary vulcanizing-practices. In any event, after the heating operation is complete, substantially no uncombined sulfur can be found in the rub- Another interesting fact with regard to my process is that much better adhesion can be obtained if a mild accelerator is used instead of a more quickly acting accelerator. The accelerator that I have found most satisfactory is methylene para- 5 toluidine, which is notably a mild accelerator. Other slow acting accelerators, such as litharge, calcined magnesium oxide, lime and mercapto .benzo thiazol without an activator, maybe used.

ess, owing apparently to the fact that the vuican .izing action is speeded up to such an extent that the rubber adjacent the interface l2 becomes vulcanized so rapidly that it interferes with the depolymerizing action of the heat applied to the metal layer l0.

When my process is carried out using a rubber stock such as indicated by any of theabove formulae, I have been able to obtain an adhesion between the rubber layer i i and an iron or steel plate I0 that is better than the adhesion possible according to heretofore known methods of first brass plating the metal and then applying the rubber. Expressed in terms of tensile strength, my process results in a bond between the rubber and metal that requires upwards of 400 lbs. per'sq. in. and over to disrupt, and usually the disruption 00- ours in the body of the rubber layer itself rather than at the interface l2.

Although, as previously indicated, it is possible to bond relatively thick layers of rubber to metal, it is sometimes preferable .to apply first a comparatively thin layer, such as the layer it, to the metal and then vulcanize onto the rubber layer ii a layer it of any desired thickness. The vulcaniza'tion of the layer M (Fig. 2) to the layer it is carriedout in the usual way.

Instead of bonding the rubber on one surface only to metal, as shown in Figs. l and 2, I also contemplate building up a laminated structure in accordance with my method by bonding an intermediate layer of rubber i5 to external metal plates l6 and l'l (Figs. 3 and 4). This can be suitably accomplished by positioning the superimposed layers Ii, i5 and B6 in a. press, mold, die or the like, and applying the heat and pressure through the containing walls that are in contact 5 with the metal plates 86 and il. Where a press is used in the manner indicated, it is found that higher pressure, in the neighborhood of several hundred pounds per square inch and over, tend to give better adhesion than where only slight pressures are used. Also, there is some advantage in using a substantially air tight press or mold, since under those circumstances air is largely excluded from the rubber during the heating operasecured "by heating the metal above the melting point of rubber and bringing the hot metal into contact with vulcanized rubber, as by inserting My process is not to be confused withpro'cesses 1933. Iam aware that many changes-may be madea metal shaft into a shell of vulcanized rubber. In my process, vulcanizable stock is used and the vulcanization is carried out simultaneously with the obtaining of the adhesion between the rubber and metal by a continued application of heat through the metal itself. It is not possible to obtain satisfactory adhesion starting with a vulcanized rubber stock in accordance with the previously known method just described. According to my method, the structural changes necessary to effect good adhesion between the rubber and metal must be allowed to take place before the mass of the rubber is vulcanized.

This application is a continuation in part of my application Serial No. 661,773, filed March 20,

and numerous details pf'construction may be varied through a wide range without departing from the principles of this invention and I, therefore, do not purpose limiting the patent granted hereon otherwise than necessitated by the prior art.

application of heat through the metal until satisfactory adhesion between the rubber and metal will be secured upon cooling.

2. The method of securing adhesion between rubber and metal, which comprises superimposing a vulcanizable soft rubber compound on a metal surface, applying sufficient heat through the metal surface to reduce the viscosity of the rubber compound locally thereof and effect a bond therebetween before substantial vulcanization of the rubber locally of said surface can occur and continuing the application of heat until satisfactory adhesion between the rubber and metal will be secured upon cooling.

3. The method of bonding; rubber directly to ferrous metal, which comprises applying suflicient heat through a layer of ferrous metal in direct contact with vulcanizable rubber stock to raise the temperature at the metal-rubber interface.

above normal vulcanizing temperatures and continuing the application of heat thereto until satisfactory adhesion between the rubber and metal will be obtained upon cooling down to normal temperature.

4. The method of bonding soft rubber directly to ferrous metal, which comprises-applying'sumcient heat through a layer of ferrous metal in direct contact with vulcanizable rubber stock containing a, .mild accelerator to raise the tempera- I ture at the metal-rubber interface above normal vulcanizing temperatures and cause depolymerlzation of the rubber locally of said *interface and continuing the application of heat thereto until satisfactory adhesion between the rubber and metal will be obtained upon cooling down to nor mal temperature.

5. The method of bonding rubber to metal, which compri preparing a layer of vulcanizable soft rubber stock containing a mild accelerator, pressing together said layer of rubber stock and a layer of metal, applying sufllcient heat to tinuing the application of heat until vulcanization s of the rubber layer is complete.

6. The method of bonding rubber to metal. which comprises preparing a layer of vulcanizable soft. rubber stock containing methylene p-toluidine, pressing together said layer of rubber stock and a layer of metal, rapidly appiyins sufllcient heat to the metal-rubber interface to raise the temperature of the rubber locallyof said interface considerably above normal vulcanization temperatures and effect depolymerization there- 1 of before vulcanization of the rubber at the in- .terface can occur and continuing the application of heat until yulcanization of the rubber layer is complete. 4

'1. The unethod ofbonding rubber to metal. 9.

which comprises forming a layer of vulcanizsble soft rubber stock containing an excess of sulphur over that'normal for vulcanization and a slow acting accelerator, arranging said rubber stock layer in contact with a layer of ferrous metal,

applying suflicient heat to saidmetal layer to heat the rubber stock adjacent said metal layer above the normal vnlcanizing temperatures to depolymerlze the said rubber stock locally of the metal before any substantial vulcanization can take place, continuing the application of heat until vulcanization of the entire rubber stock layer is substantially complete.

8.- The method of bonding rubber to metal, which comprises pressing a layer of vulcanizable soft rubber stock .against a layer of ferrous metal, applyingsteam of at least 100 lbs. per sq. in. pressure to said metal layer to rapidly raise the temperature of the rubber adjacent the metal surface and cause the rubber to depolymerlze be- 40 fore, vulcanization sets in and continuing the application of heat until good adhesion between the ruober and metal will be obtained upon cooling.

9. The method of bonding rubber to metal, which comprises pressing a layer of vulcanizable soft rubber stock containing a mild accelerator directly against a layer of ferrous metal, applying steam of at least 100 lbs. per sq. in. pressure to said metal layer to,rapidly raise the temperature of the rubber adjacent the metalsnrfsce and cause the rubber to depolymerlze before vulcanization sets in and continuing the application of heat until good adhesion between the rub ber and metal will be obtained upon cooling and until substantially complete vulcanization of the rubber stock has been obtained.

10. The method of bonding rubber to metal, which comprises pressing a layer of vulcanizable soft rubber stock containing, a mild accelerator directly against a layer of ferrous metal, applyiing steam of about 135 lbs. per sq. in. pressure to said metal layer to rapidly raise the temperature of the rubber adjacent the metal surface and cause the rubber to depolymerize before vulcanization sets in and continuing the application of heat for at least one hour-until good adhesion between the rubber and metal will be obtained upon cooling and until substantially complete vulcanization of the rubber stock has been obtained.

' v '11. The method of bonding rubber to metal,

which comprises pressing metal plates against opposite sides of a layer of vulcanizable soft rub- 15 ber, applying heat rapidly through said metal plates to raise the temperature 0! the rubber adjecent the feces of said metal plates substerr tielly above normal vulcanizing temperatures, thereby causing physical adhesion of the rubber to the metal, and continuing the application of heat until vulcanization is substantially complete. 12. The metlwol of bonding rubber to motel, which comprises pressing; mete! plates against opposite sides or c layer or vulceulzeble sort rubrm coutclniur as slow coring elccelerctor apply mg leect racially through said metal plates to raise the temuercture oi the rubber ucljeceut 'fsces or selrl motel pletes substsutiully ebove normal rules lies temuereuu'es. thereby cruis mg 'gslayslcel all scion of the rub oer to the motel, curl coutiuumu r e upgrllcuticn of heat until mm ceuiretlou is sulrstcntislly complete. 13 The melllocl of bonding nrbber to m which comprises pressing motel u opposite eicles or e. lcyer muse-r relele so? her, applying heel; or ruesm of at 163% through c. pistes to raise tempereture of rubber urlleccut the feces or" sold metsl'pletes subeteze tielly shove normal vulcc-nlzlug temuersturee, thereby causing physical adhesion of the rubber to the motel, and continuing the cuplicctiou of best until wlcsnizetion is substcutisslly complete.

14. A composite article comprising a layer or is. The method or attaching rubber to motelcouoprising applying a slab of urivulcsnlzed rubher to a. surface thereof, and subjecting the motel to a temperature in excess of that reouirecl for vulcemzstion and for c, sumciently long period to secure intlmete bonding of the rubber with the metal while leaving the my of the rubber unexposed to the high temperature.

sture in excess of vulcenizing temperature and ilor u sumcientiy extended period to secure iuti= mete bonding of the rubber with the metel while the metal iserponded.

18. The method of attaching rubber to motel comprising applying to the metal a. slab of unvulcanized rubber so that the heat will flow freely therethrough, subjecting the surface of the rubher in contact with the metal to a, temperature in excess of vulcanizlng temperature and for a. sumciently extended period to secure intimate bonding of the rubber withthe metal while the metal is expanded, and during the process maintaming the rubber remote from the surface in contact with the metal below vulcauizing tompereture. I

19. The method of bonding rubber to metal comprising applying to e. surface or the metal 9. slab of unvulcauized rubber containing sulphur and of sumoleut thickness to permit the dissipation of heat through. the rubber, rosinteining the outer surface or the rubber at as temperature below vulcanization and raising and comprising applying to a. surface or the motel er slab of unvulcenized rubber containing sulphur and of sumcient thickness to permit the dissipotion of heat through the rubber, memtslning the main body or the rubber at c temperature belowmlcsmlmtion, raising and maintaining the temperature at the interracial suriuce er the rubber and. metal above the temperature of vul cenizetlon and thereby securing depolymerizetime of the rubber out the union while driving the sulphur ewsy irons the motel surface, and uniting the slob of rubber to s second body of rubber.

21. The method of? bonding uuvulcenised rubher to metal comprising subjecting the rubber in contact with the metal to temperatures in excues cl Wlecnising temperatures so that the surface of the rubber while in contuct with the mete! is deuolrmerizeo.

22. The metlocd oi bonding a. layer of unvuloeruzerl rubber to metal comprising subjectin the rubber in coutect with the metal to temperuturee in excess of vulcenlzmg temperatures so that the surface or the rubber while in contect with the motel is depolymerlmd.

23. m method of bonding a layer or unvulcsulzw rubber to metal comprising subjecting the rubber in contact with the metal to temperstures m excess of vuloeuizing temperatures so user the surfoce or the rubber while in con- 55 tect with the motel is rlepolymerieed, end there-- otter vul ew-1' u Mr or rubber thereto. 

